David Cameron believes a Conservative government would have no more than six months to lay down the drastic measures needed to tackle Britain's £163bn fiscal deficit amid fears that any goodwill from the electorate would start to fade by the end of the year, senior Tory sources have told the Guardian.

In a sign of the depth of preparation by the party leadership, senior Conservatives said Cameron believes that he must spell out the depth of crisis in Britain's public finances by November and the scale of the spending cuts that will have to be introduced next year.

Cameron's plans will apply if he wins an overall parliamentary majority – with at least 326 seats – or if the Tories are substantially the biggest party in the House of Commons.

A Guardian/ICM poll today indicates that Cameron would lead the largest party in a hung parliament. The poll puts the Tories on 33%, down three points on an ICM/Sunday Telegraph poll. Labour and the Liberal Democrats are in joint second on 28%. The findings may do something to check the rightwing media's narrative that Cameron is pulling away as election day looms.

Senior Conservative sources said if Cameron failed to secure an overall majority he would attempt to follow the example of the SNP leader, Alex Salmond, who leads a minority administration in Holyrood. A Tory government would set out its plans and "dare" Labour and the Liberal Democrats to provoke a second election this year by voting down the Queen's speech on 25 May or an emergency budget in July.

Amid signs that the Tories are failing to build a decisive lead, Cameron today reassured wavering voters that frontline services would not be cut.

Pledging to act in a "big-hearted way", Cameron told BBC1's Andrew Marr Show: "What I can tell you is any cabinet minister, if I win the election, who comes to me and says: 'Here are my plans' and they involve frontline reductions, they'll be sent straight back to their department to go away and think again. After 13 years of Labour, there is a lot of wasteful spending, a lot of money that doesn't reach the frontline."

But one senior Tory said: "David and his team are obsessed with avoiding the vacuum into which Tony Blair was sucked after winning in 1997. They know that, if we win, this is going to be a deeply unpopular government. "They have six months at maximum to create the parameters for how they will implement the tough decisions. Unpleasant decisions will have to be made. It is only in the excitement of the general election aftermath that you can do that." The sources identified three events in the first six months of a Tory administration to show Cameron is serious about delivering change and being honest about the inevitable "unpleasantness" ahead:

• An emergency budget by George Osborne within 50 days of the general election to give details of the £6bn of spending cuts to be introduced this year and the overall reduced "spending envelope" for next year.

One shadow minister said: "It will be as tough as you can imagine. It will be Osborne's political judgment to decide how hard the pips can be squeezed. You would hope that within three years, by which time the economy would have recovered, you could see the sunny uplands. You would then be able to say it had all been worthwhile."

• A spending review in the autumn to set out spending department-by-department in the next financial year starting in April. Cameron pledged that NHS spending would rise at least in line with inflation. But he admitted that Tory cuts for next year, such as introducing a pay freeze for 80% of Britain's public sector workers, do not go far enough.

• Plans to allow existing schools to change their status and become free schools would be on the statute book by the start of the new academic year. This is designed to show that radical change would be delivered by a Cameron government.

In a damning piece for the Guardian's Comment is Free site today, Mona Sahlin, the Swedish Social Democratic party leader, says her country is about to abandon the school reforms the Tories want to install in Britain. Unusually intervening in a foreign election, Sahlin says: "The free school system, implemented without imposing clear standards, has seen schools opening in sub-standard facilities, often without libraries, and with a far greater number of unqualified teachers.

"What's more, the introduction of free schools has led to increased segregation where pupils from the same social background increasingly concentrate in certain attractive free schools.

"Creating a free market as the Conservative proposals do without providing the funding to allow for the surplus capacity you need will certainly harm standards."

Gordon Brown visited 10 London constituencies today eventually leaving a pub in Kilburn by a side door after it was besieged by Tory and Lib Dem activists. He is now running a campaign almost exclusively warning of Tory cuts, and claiming that the party's first tax cut would be for Britain's richest millionaires.

Brown also described the Tory manifesto as a "horror show",, flagging up their plans on inheritance tax, public services and the deficit. adding: "I say the Conservative party is not fit for government if it has policies like that."

A YouGov daily tracker poll for the Sun today shows the Conservatives unchanged on 34%, five points ahead of the Liberal Democrats who were up by one on 29%, with Labour unchanged on 28%.